1. Field of the Invention
The invention relates to a circuit and a method for an integrating amplifier which increases the input range of the charge that can be measured, and more particularly to an integrating amplifier on an IC where an internal feedback loop is added thereby eliminating the need for a another external capacitor and pad.
2. Description of the Related Art
To measure charge and thus capacitance, an amplifier wired as an integrator could be used. The charge to be measured is injected into the input of the amplifier and from there is fed, via an integrating capacitance, to the output of the amplifier. The measurement is initiated by a current pulse on the input of the integrator. The integrating capacitance, connected between the negative input and the output of the amplifier, collects the charge transferred by the input pulse. Because of the limited output swing of the amplifier the amount of the charge that can be measured is limited. In current designs the range for the charge to be measured is changed by changing the integration capacitance. In integrated circuits, where an external device is used as integration capacitor, this method requires not only an additional capacitor but an additional pad. This is a disadvantage for designs which need to be placed inside small packages and where pads are either at a premium or not at all available.
Referring now to FIG. 1, we describe a circuit 10 of the current practice requiring an additional pad for a second external integrating capacitor. Circuit 10 comprises an amplifier 11a having a minus (−) and a plus (+) input, and an output OUT. The minus input is coupled via pad 11b to input IN which provides current pulses Q=IINt from a current pulse generator 15. The plus input is coupled to a reference voltage VREF, for example GROUND. Output OUT, providing current IOUT and voltage VOUT, is coupled via pad 12b, an external integrating capacitor 12a (CINT), and pad 12c to the minus input of amplifier 11a. Output OUT is also coupled via switch 14 (S1), additional pad 13b, a second external integrating capacitor 13a (CINT*), and pad 12c to the minus input of amplifier 11a. 
In an ordinary integrator, charge is transferred to the capacitance to be measured by a voltage pulse causing an integration current to flow. The amplifier delivers this current by adjusting its output voltage. The charge on the capacitor CINT is equal to the charge introduced by the current pulses at input IN:Q=IINTtresulting in an output of:
      V    OUT    =                    I        INT            ⁢      t              C      INT      
To measure a larger charge within the given output voltage range in current designs, the integration capacitor needs to be adjusted, that is another external capacitor needs to be added. The disadvantage of this circuit is that it needs an additional pad and that other external capacitor, leading to a higher module cost and a bigger die area. There is presently no known way to avoid this problem. The below described invention is directed to a circuit arrangement which offers a novel solution.
U.S. Patents which Relate to the Present Invention are:
U.S. Pat. No. 6,608,516 (Lennous) discloses a system and method for adjusting the time constant of an integrator. In one embodiment, a variable time constant integrator includes an amplifier, a capacitor, and a variable gain element. In another embodiment, the variable gain element may include a MDAC (Multiplying Digital to Analog Converter). The Patent refers to signal conditioning and uses a voltage feedback to limit the range.
U.S. Patent Application 2007/0229161 (Killat) teaches methods and circuits for a low noise and high linear voltage-to-current converter which requires only small integration resistors. The circuit uses a shunt to measure the current, where a current IDC used as feedback is static and is derived from a bias voltage. Current IDC is used to set the DC operating point.
U.S. Pat. No. 4,059,812 (Proctor) describes a phase-locked-loop which comprises an adjustable integrator further comprising an integrating amplifier and an adjustable voltage source applied to one integrating amplifier input. The output of the integrating amplifier changes the set time of one or more one-shot controls feeding back to the adjustable integrator and thus to the integrating amplifier.
It should be noted that none of the above-cited examples of the related art have the features and advantages of the invention described below.